1. Field of the Invention
The present invention relates to a storage medium including an actuator employing a ramp loading/unloading technique, and a method of controlling movement of the actuator.
2. Description of the Related Art
In the recent years and continuing, technologies have been developed at an accelerating rate to increase recording density of storage media, such as hard disks, and the recording density per area is increasing by 100% annually. One subject in this development is to improve reproduction performance.
In a hard disk, a magnetic head records or reproduces data in a magnetic disk while floating over the surface of the magnetic disk at a certain height. The amount of space between the magnetic head and the magnetic disk is called “floating height”, or “flying height”, or “head gap”. It is known that the reproduction performance improves when the floating height decreases, and as the state of the art of the technology, a floating height as small as 10 nm has been achieved. To achieve such a small floating height, the surfaces of the magnetic head and the magnetic disk have to be made smooth.
In the related art, the magnetic head is operated in a CSS (Contact Start Stop) mode, in which the magnetic head comes to rest on the surface of the magnetic disk when the drive is not in operation, and the surface of the magnetic disk has to be textured (roughened) to prevent adhesion of the magnetic head to the surface of the magnetic disk to prevent impact on rotation of the magnetic disk.
However, as mentioned above, to achieve a smaller floating height, the surfaces of the magnetic head and the magnetic disk have to be made smooth to improve surface perfection, and due to this, adhesion of the magnetic head to the surface of the magnetic disk becomes remarkable.
One of the solutions to this problem is the so-called ramp load/unload technique, in which the magnetic head is moved away from the surface of the magnetic disk to be laid on a ramp when the disk is not rotating.
With the ramp load/unload technique, surfaces of the magnetic head and the magnetic disk can be made smooth. Further, because the magnetic head and the magnetic disk are not in contact when the drive is not in operation, resistance against shock of the drive is highly improved. For example, even when one moves around while carrying a personal computer, shock to the computer can be suppressed, and trouble can be reduced. Because of these benefits, the use of the ramp load/unload technique is wide-spread.
FIG. 1 is view for schematically explaining the ramp load/unload technique.
In the ramp load/unload technique, as shown in FIG. 1, when unloading a magnetic head 100, the following operations are performed.
As shown in FIG. 1, an actuator 102 supports the magnetic head 100 floating over a magnetic disk 101 in operation. First, the actuator 102, which is at a position A, moves in the right direction in FIG. 1. When the actuator 102 moves to a position B, a lift tab 103 formed in the actuator 102 comes in contact with a ramp 104 located near the magnetic disk 101, and the actuator 102 is lifted up along the slope of the ramp 104. When the actuator 102 moves further in the right direction to a position C, the magnetic head 100 is moved beyond the outer diameter of the magnetic disk 101.
At the position C, because the magnetic head 100 and the magnetic disk 101 are not in contact, even when a shock or any outer force is imposed on the magnetic head 100, contact of the magnetic head 100 with the magnetic disk 101 can be avoided.
When loading the magnetic head 100, the above operations are performed in the reversed order. That is, the actuator 102 moves from the position C to the position B and finally the position A. Because of an air bearing formed between the magnetic head 100 and the magnetic disk 101, the magnetic head 100 can stably float over the magnetic disk 101.
When the magnetic head 100 is raised (unloading operation) with the air bearing existing between the magnetic head 100 and the magnetic disk 101, or when the magnetic head 100 is lowered down to the magnetic disk 101 (loading operation) with the air bearing to be formed, while the ramp 104 raises the lift tab 103 upward, the actuator 102, which holds the lift tab 103, is engaged with a springy suspension and tends to move downward. Therefore, for example, in the unloading operation, if the upward speed of the magnetic head 100 is not sufficiently high, the magnetic head 100 may be pulled down to contact the surface of the magnetic disk 101. Similarly, in the loading operation, if the downward speed of the magnetic head is too high, the same problem may occur.
To solve this problem, it is proposed to control the moving speed of the actuator to be in an appropriate region so that the vertical speed of the magnetic head 100 relative to the magnetic disk 101 is in an appropriate range, thereby, preventing contact of the magnetic head 100 with the surface of the magnetic disk 101.
Specifically, the vertical speed of the actuator 102 is determined by the shape of the ramp 104, and the horizontal speed of the actuator 102 is related to a voice coil motor (VCM) that drives the actuator 102.
FIG. 2 is a graph showing variation of the speed of the actuator 102 in feedback control in the related art, in which the bandwidth of the feedback control is fixed.
In FIG. 2, in the unloading direction, (the right direction), the speed of the actuator 102 is expressed by a negative value. For example, an increase of the speed of the actuator 102 in the unloading operation corresponds to an increase of the graph in the downward direction in FIG. 2.
When the lift tab 103 comes into contact with the ramp 104 at the position B, the moving speed of the actuator 102 drops notably, as shown by the solid line in FIG. 2. Accordingly, the vertical speed of the magnetic head 100 decreases remarkably, and the magnetic head 100 may contact the surface of the magnetic disk 101.
As a solution to this problem, it is proposed to detect the decrease of the speed of the lift tab 103 when the lift tab 103 comes into contact with the ramp 104, or detect an increase of a control variable in control of the voice coil motor, and increase a gain of the feedback control or add a feed-forward control variable according the detection results. Thereby, the speed decrease can be suppressed.
For example, Japanese Laid-Open Patent Application No. 2001-052458 discloses such a technique.
In this technique, as shown in FIG. 2, a threshold value of speed is used for detecting the speed decrease, and it is required that the threshold of speed be set sufficiently far away from a target speed so as not to make unnecessary response to even a small speed change caused by external shock or vibration. With such a threshold, however, the detection time Δt increases. Here, the detection time Δt is defined to be the time period from the time when the lift tab 103 comes into contact with the ramp 104 to the time when the contact is detected by detecting the decrease of the moving speed. During the detection time Δt, measures cannot be taken to compensate the decrease of the speed, and thus it is difficult to suppress the speed drop, as shown by the solid line in FIG. 2.
On the other hand, for convenience of usage, it is required that the loading and unloading operations of the magnetic head 100 be completed in a short time. Hence, it is necessary to shorten the detection time and increase the moving speed of the actuator.
However, if the threshold value of the moving speed is set close to the target value so as to shorten the detection time Δt, detection errors may occur, and this may cause unintended large changes of the speed.
If the gain in the feedback control is set higher, oscillation may be induced easily, and this may degrade the stability of speed control. For example, when the device is being carried or used in a vibratory environment, such as in a train, operational stability of the device cannot be secured.